CS 15-212: Fundamental Structures of Computer Science II

Lecture 21: Type Checking

Lexical analysis converts ("transduces") the input character stream into a stream of tokens. The syntactic parser then converts the token stream into a stream of expressions, or "abstract syntax" trees. The next job of the interpreter is to carry out type checking to make sure that each expression is well-typed. A precise specification of the typing rules can be given in terms of a deductive system, expressed in terms of a set of inference rules. The type checker traverses the syntactic parse tree (typically in postorder), building up type environments and applying the inference rules to assign a type to each node in the tree. We illustrate the basic idea on our fragment of ML consisting of arithmetic expressions together with Booleans and conditional statements.

Key Concepts

Sample Code


On-line Notes

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John Lafferty